1. Field of the Invention
The present invention relates to a method for controlling the solidification segregation of steel, particularly for mitigating the solidification segregation which causes defects in steel products obtained by continuous casting, etc., more particularly for effectively controlling inter-dendritic segregation during the solidification of steel.
2. Description of the Related Art
The segregation of solutes during continuous casting results in formation of surface flaws and cracks of the casting, thereby impairing the qualities of the final product. Mitigation of the solidification segregation has therefore been desired. Known methods for mitigating the segregation includes: adding calcium into the molten steel; preliminarily decreasing, by refining of the molten steel, the amount of solute elements which cause detrimental segregation; and lessening the roll-distance of a continuous casting machine to suppress the bulging of a casting and electromagnetically stirring the melt to mitigate the central segregation.
From the viewpoints of saving energy and labor, however, either direct rolling or hot-charge rolling is preferable.
In direct rolling, the casting is not allowed to cool down to room temperature, but is rolled directly after the continuous casting. In hot-charge rolling, the casting is charged in a heating furnace before cooling to room temperature and is then rolled.
Japanese Unexamined Patent Publication (Kokai) No. 55-84203 proposes a method for suppressing the surface cracks in direct rolling and hot-charge rolling. The method proposed by this publication involves subjecting the casting, after its melting and solidification (the primary cooling), to ultraslow cooling during a secondary cooling stage until the initiation of the hot-rolling.
This publication threw light, by a simulation experiment, on a particular temperature range of from 1300.degree. C. to 900.degree. C. wherein elements, such as phosphorus, sulfur, oxygen, and nitrogen, detrimental to the hot-workability of steels segregate and precipitate as non-metallic inclusions, and drew attention to the fact that surface cracks frequently occur when the percentage of reduction in area of steel materials becomes less than 60%. The method proposed in this publication controls the morphology of the above-mentioned elements precipitated as non-metallic inclusions so as to suppress the hot-cracking of castings.
Japanese Unexamined Patent Publication No. 55-109503 and No. 55-110724 also disclose to slowly cool the continuous castings prior to the hot-rolling and to directly roll them.
Japanese Examined Patent Publication (Kokoku) No. 49-6974 discloses a cooling and heating treatment of a continuously cast strand in which the temperature difference between the surface and central liquid of the castings is kept from becoming excessively great.